There are three different ways to express future events: WILL, BE GOING TO, and the present continuous tense, each of which expresses a different meaning.
WILL vs BE GOING TO for future intentions
You can use WILL and BE GOING TO to talk about things that you want to do in the future. However, the meanings are different. WILL expresses spontaneous offers, promises, and decisions. That means your decision, promise, or offer to do something is made at the time of speaking. BE GOING TO expresses offers, promises, or decisions that have been made before the time of speaking.
Don’t worry. I will take you home.
You say this to a friend who seems to have no means of transport for him or her to go home after a party.
I’m sorry I can’t go with you tomorrow. I am going to take my parents to the railway station.
You can’t go because you have already decided to take your parents to the railway station before the time of speaking. Your intention has been decided before your friend asks you to come with him or her.
WILL vs BE GOING TO for future predictions
WILL expresses predictions that are based on personal judgement or opinion. There is no evidence or sign that you can use to make predictions. BE GOING TO expresses predictions that are based on signs or evidence you see that makes you sure things are going to happen in the future.
I’m sure you will have a good time in Chicago. (Your prediction is based on your own opinion)
Dark clouds are hanging over the city. It is going to rain soon. (You say this because you see present evidence or sign – the dark clouds)
BE GOING TO vs THE PRESENT CONTINUOUS for future plans
BE GOING TO expresses future intentions that have been decided but not fixed. THE PRESENT CONTINUOUS is used to express fixed future plans. Compare these two sentences and see the difference in meaning.
My parents are going to visit me. (it has been decided that your parents are going to visit you, but they have not bought any plane tickets yet)
My parents are visiting me next week. (Your parents’ plan to visit is fixed. They have arranged everything for this visit)
Now let’s take a look at the following three sentences:
I WILL return the book to the library now. (your decision to return the book is spontaneously made at the time of speaking. Perhaps, you are not aware that today is the deadline. Somebody has just reminded you).
I AM GOING TO return the book to the library soon. (you already have a plan to return the book and you have made this decision before the time of speaking)
I AM RETURNING the book to the library tomorrow. (you have a fixed plan to return the book, and you have arranged everything, including when and how you are returning it)